Enhancement of exciton–exciton interaction at the interface of n–n type organic solar cells by interfacial engineering

Abstract

The generation of free carriers in organic solar cells (OSCs) is very important. Free carriers in OSCs are generally contributed by the dissociation of excitons at the donor/acceptor interface. An n–n type device formed by two n-type materials was designed, and a new method was proposed to generate free carriers through the exciton–exciton interaction at the interface. However, the n–n type device shows poor performance because of weak exciton–exciton interaction at the interface. The interface design and optimization of the n–n type device will considerably influence and promote exciton–exciton interaction and ultimately allow the contribution of photocurrent to be determined. In this work, an effective Tris-8-hydroxy-quinolinato aluminum (Alq3) interface layer was introduced into the active layer of an n–n-type OSC to explore the exciton–exciton interaction that occurs at the perfluorphthalocyanine zinc/fullerene interface. The effect of various Alq3 thicknesses on the performance and external quantum efficiency (EQE) of devices was systematically investigated. The best performance for these n–n type OSCs was achieved at the 5 nm-thick Alq3 interface layer. The analysis of EQE measured under bias light further demonstrated that free carriers were contributed by exciton–exciton interaction. The EQE measured under different frequencies illustrated that the interaction of interfacial exciton–exciton was enhanced by the introduction of Alq3. A large displacement current was also generated in the Alq3 based on an n–n-type device.